It has been known to measure an electric current in a potential-free manner by the magnetic field that the current produces. To determine the strength of the magnetic field, a semiconductor element that produces a voltage output more or less proportional to the magnetic field to be measured may be used. Such semiconductor devices include Hall effect devices and magneto-transistors. One problem with such devices is that there may be fluctuations in the transfer factor (i.e. transfer characteristic) between the magnetic field to be measured and the output voltage. In order to suppress the influence of such fluctuations in the transfer factor of the magnetic field sensor, a magnetic counter-field may be originated by means of a magnetic coil with magnetic counter-field virtually wholly compensates (i.e. cancels) the magnetic field to be measured (DE-AS No. 26 21 302). One disadvantage of such compensated transducers is the relatively high performance requirement needed to originate the compensating magnetic field.
In Swiss patent application No. 4,448/83-2, a circuit is described in which an auxiliary magnetic field of known magnitude is superimposed on a magnetic field to be measured by means of a magnetic coil. The transfer characteristic curve of the magnetic field sensor is determined solely by the portion of the auxiliary magnetic field in the output signal. The output signal of the magnetic field sensor is divided by the thus determined slope to calculate the magnetic field.
This invention has the object to create a circuit of the afore-described type that is economical to operate and does not have a high performance requirement for originating a compensating auxiliary magnetic field, and wherein no determination of a slope of the transfer characteristic of a magnetic field sensor is required. It is a further object of the present invention to provide a circuit which compensates for fluctuations in the transfer characteristic (i.e. the transfer factor) of a magnetic field sensor.